The boy Mark ... adventurer and would-be playboy

If he were not such an obviously unappealing character, Mark Thatcher could almost be the rather dashing anti-hero of a Frederick Forsyth novel. The former racing driver turned wheeler-dealer, the fixer who moves among the world's wealthy and powerful and was once reckoned to be worth £40 million.

Almost, but not quite. Instead the image he presents is of a deeply flawed character: arrogant, surly, aloof and not terribly bright. He failed his accountancy exams three times, and at Harrow was nicknamed "Thickie". The inevitable implication is that if it were not for his surname he would never have been able to make a fraction of the wealth he has.

Yet none of the more serious allegations that have been levelled at him over the years - accusations about sleazy deals or dodgy businesses - has been made to stick.

He has, however, remained an almost constant embarrassment to his mother. One of his earliest and most infamous escapades was when he got lost in the Sahara desert for six days in 1982 during the Paris-Dakar rally. From the moment he greeted the world's media with Bruce Forsyth's catchphrase - "Nice to see you, to see you, nice" - it was an episode which cemented in the public imagination two fundamentals of Mark Thatcher's character: his incompetence, and his arrogance.

There was astonishment among the assembled photographers when he refused to shake his father Denis's hand on the grounds that he had never done such a thing in his life - and even more when he resisted shaking the hand of the Algerian army officer who led the rescue mission on the grounds that he had not actually been lost at all.

Thatcher, who inherited his late father's baronetcy last year, left Harrow in 1971 with three O-levels. As well as his abortive attempt to become a chartered accountant, he went through a series of jobs which each lasted about a year, dabbled in the Hong Kong business world and even tried his hand at being a racing driver but repeatedly crashed cars. In 1977 he set up Mark Thatcher Racing, which developed cash problems.

Using his mother's name to his own advantage once she was prime minister led him into embarrassing scrapes. The first came when, using a consultancy firm he had set up called Monteagle Marketing, he was said to have helped the Cementation company win a multi-million-pound contract to build a university in Oman. He had flown to Oman the day after his mother arrived there for an official visit, and it was claimed the events represented a conflict of political interest.

More serious was the Saudi arms affair. Thatcher was said to have received a fat commission - up to $20million - on the back of the £20 billion Al Yamamah arms contract between the UK and Saudi Arabia while his mother was in Downing Street, and although no solid evidence has ever been found to support the claim, neither has there ever been a convincing explanation as to what he was doing mixing in defence circles at the time of the deal.

Thatcher's evasions have not helped, either. Asked about whether he has ever been an arms dealer, his favourite circumlocution is: "I've never sold a penknife. I may have sold a letter-opener."

In 1987 he married Diane Burgdorf, the daughter of a millionaire Texas car dealer he met while working as a salesman for Lotus. Settled in Dallas, he created a complex web of companies, helped along by the contacts he had made through his mother. His ability to alienate people also continued. Neighbours were left aghast at the occasion he stormed into a house across the street and ordered the woman who opened the front door to move her car from outsidehis house. At a get-to-know you party he refused a name tag, saying: "If they they don't know who I am by now they never will."

But he also made friends. Intriguingly, a 1994 newspaper article reported among his powerful Texan allies were one George W Bush Jr, then running for state governorship.

By then he was said to be worth as much as £40million. But he told the Financial Times in 1994 his net worth was no more than ?5million. His US business dealings soon caused him major headaches. The security alarms company Emergency Networks, of which he was a non-executive director, went bust, with the US Internal Revenue Service claiming it owed £1.7million in unpaid taxes. He was later cleared of any liability.

Then he was sued by a former partner in a Texan fuel company who accused him of conspiracy, fraud, money laundering, usury, deception and perjury. He later settled the matter with an unconfirmed out-of-court payment said to be £330,000.

In 1995 he moved to Cape Town - not least to escape what he called the "Texas crap" - with his wife and their two children, Michael, now 15, and Amanda Margaret, 11. Since then there have been persistent rumours the marriage is in trouble, although the couple have never officially separated. Even once settled in a £570,000 mansion, Thatcher could not escape embarrassment and controversy.

In 1998 his firm was investigated over claims he had been running an alleged "loan-shark" operation among South African government officials. At one stage he was said to have lent money to 900 policemen, soldiers and other public servants.

Now his arrest over allegations he was involved in a planned coup in Equatorial Guinea seems the latest in a line of unsavoury controversies.

His twin sister Carol, recalling the time he was lost in the desert, wrote in her autobiography: "The episode did have one benefit. We could relax a little, for Mark had hung an ' occupied' sign on the family's ' embarrassing relative' slot."

All these years later the sign still seems to be hanging there. Only time will tell whether this is just another embarrassment - or something rather more serious.